How to Build a Hanging-shelf Built-in Bookcase

Shelfology sells welded steel brackets that screw into wall studs and then stick out of the wall 8-10".
I bought 26 of the 10" brackets. 4 for each shelf plus an extra 2 for the bottom shelf where I put heavier books.
Each bracket is rated to hold 100 pounds though they are easily strong enough to hold anything I put on the bookcases without bending.
In the first photo, you can see what the brackets look like poking out through the wall. 3/4" holes were drilled in the right spot in the drywall.

For the first row, I measured 17" off the floor and found the studs behind the wall with a stud finder. I put small pilot holes just to the side of the studs at the right height.

The only drawback to this method is that you have to destroy the drywall on the other side. In my case, the other side of the wall was in the garage so it wasn't an issue. Once I knew where the brackets were going to have to be installed (from seeing the pilot holes on the other side), I cut the drywall off - about a 8' x 10' section. I pulled back or removed much of the insulation. From there, I could easily access the studs and see where the holes needed to be. Then I drilled 3/4" final holes from the garage side at the exact spot (about 1/2" off from the edge of the stud). I could then push the bracket through the hole and secure it temporarily to the stud with clamps.
Then on the finished side of the wall, I checked each mount with a right angle to make sure it was parallel to the floor and not pointing too much to the left or right. Adjustments could be made if too much to the left or right by putting washers between the stud and the bracket in the right places.

On the stud side of the wall, I drilled 1/4" holes through the bracket's holes all the way through the stud. Then 1/4" (#20) x 2-1/2" hex head bolts were pushed through, capped with large washers on the far side of the stud, and tighted with 1/4" nuts with a socket wrench. Each bracket has 4 of these.

Create the wood wall supports that go over the steel rod mounts

With a collection of 2"x2" oak cut to about 11" long, I had to drill 3/4" holes about 8" deep. Those blocks then slid over the metal rods to make it look like the final shelves were supported by nothing but 4 small wood ledges.
Drilling these blocks was probably the hardest part of the project.
I had a large drill press to get started, a WEN 12", which is 36" tall and has a 3" travel. It was barely large enough. I was able to use it to drill a 1/8" pilot hole straight down, but that required a couple of attempts since I could only drill 3" down.

Utlimately the drill press was useful for getting the pilot holes started straight down. All of the drilling after that I simply clamped the wood pieces into a vice and hand-drilled with a corded, and sometimes, coredless dril.
Once the pilot holes were in place, the hand-held drills kept close to its location so all of the final holes came out also straight down.
Because oak is a hardwood, I had to buy 12" long drill bits in every size stepping from 3/8" up to 3/4".

Once I had one row of brackets in the wall, I knocked the wood coverings on it with a mallet and put a 12" wide oak shelf over it to test the fit. I marked on the wood coverings where the shelf came to and cut them all down a bit.
They look best when they are about 1/8" shorter than the shelf. Choosing the straightest and best looking planks for the bottom 4 shelves is important. I had a couple of slightly warped boards at the top and needed to put a couple of screws
in at the end to secure the shelf to the wooden coverings, but because they are above eye-level, you can't see the screws. Nowhere in the bookcase, in fact, can you see any screw, nail, or join.

(3) Measure, cut, stain, abd install the the shelves

The shelves and the wood supports were sanded with 220 grit orbital sander, pre-conditioned, and then stained three times with a dark walnut. I only used beeswax at the end to finish them.
I did use wood glue to secure the shelves to the wood brackets because you don't want someone to pull on a shelf and have it come off.
I had to use a bit of WD-40 in some places to get the wood support over the steel mouning rod; this did leave a small stain at the end of the wood, but you can't tell because of the dark stain.
In one place where the mounting bracket was >12" from the wall and I had some heavy books, the shelf sank a bit so I simply glued it to the wall - it worked find and you can't see the glue.
And of course I had to replace and repair the wallboard on the other side.